A team of Japanese scientists created a tiny rabbit sculpture, the size of a typical bacterium, using a new 3D printing material, a new type of resin which can be molded into complex, highly conductive 3D structures. Potential applications of the new photopolymers include 3D carbon electrodes for fuel cells or batteries and interfaces for biosensors.

This rabbit sculpture is one of several whimsical shapes created by the team. The work opens a door for researchers trying to create conductive materials in almost any complex shape at the microscopic or cellular level.

The research team, which includes physicists and chemists from Yokohama National University, Tokyo Institute of Technology, and the company C-MET, Inc., presents its results in a paper published in the Optical Society's (OSA) open-access journal Optical Materials Express.

"One of the most promising applications is 3D microelectrodes that could interface with the brain," says Yuya Daicho, graduate student at Yokohama National University. These brain interfaces, rows of needle-shaped electrodes pointing in the same direction like teeth on combs, can send or receive electrical signals from neurons and can be used for deep brain stimulation and other therapeutic interventions to treat disorders such as epilepsy, depression, and Parkinson's disease.

"Although current microelectrodes are simple 2-D needle arrays," Daicho says, "our method can provide complex 3-D electrode arrays" in which the needles of a single device have different lengths and tip shapes, giving researchers more flexibility in designing electrodes for specialized purposes.

Currently, the commercially available resins that work best with modern 3D printing techniques do not respond to carbonization, a necessary part of the electrode preparation process. In this stage, a structure is baked at a temperature high enough to turn its surface to carbon. The process of "carbonizing," or charring, increases the conductivity of the resin and also increases its surface area, both of which make it a good electrode. Unfortunately, this process also destroys the resin's shape; a sphere becomes an unrecognizable charred blob. What researchers needed were new materials that could be crafted using 3D printing but that would also survive the charring process.

The Japanese team, led by Daicho and his advisor Shoji Maruo, developed a light-sensitive resin that included a material called Resorcinol Diglycidyl Ether (RDGE), typically used to dilute other resins but never before used in 3D printing. The new mixture had a unique advantage over other compounds – it was a liquid, and therefore potentially suitable for manipulation using the preferred 3D Stereolithography methods.

Daicho, Maruo, and colleagues tested three different concentrations of RDGE in their new compounds. Though there was shrinkage, the materials held their shapes during the charring process (controlled shrinkage of a microstructure can be a good thing in cases where miniaturization of a structure is desired). The resin with the lowest concentration of RDGE shrank 30 percent, while that with the highest concentration shrank 20 percent.

This shows two microstructures made with the new material, containing the highest concentration of RDGE. Left: Pre-charring. These pyramid and bunny models did not respond to the preferred method of 3-D shaping, so they were created using an alternative process. Right: Post-charring. Notice that the pyramid and bunny shrink significantly less than those made from the material with a lower concentration of RDGE. Credit: Optical Materials Express.

The researchers tested also their new resin's ability to be manipulated using UV light and laser beam. In one technique, called microtransfer molding, the light-sensitive liquid was molded into a desired shape and then hardened by exposure to ultraviolet (UV) light. The other technique, preferred because of its versatility, made use of the liquid resin's property of solidifying when exposed to a laser beam. In this process researchers used the laser to "draw" a shape onto the liquid resin and build it up layer by layer. Once the objects were shaped, they were carbonized and viewed with a scanning electron microscope (SEM).

This shows two microstructures made with the new material, containing the lowest concentration of RDGE. Left: Pre-carbonization. This circular table and bunny models were formed using two-photon polymerization. Right: The same models after they have been carbonized, or charred. Notice that they shrink significantly but maintain their shape.Credit: Optical Materials Express.

"When we got the carbon bunny structure, we were very surprised," Maruo says. It was exciting, he continues, to see that "even with a very simple experimental structure, we could get this complicated 3-D carbon microstructure." The rabbit's shape would be much more difficult, expensive, and time-consuming to create using any of the existing processes compatible with carbonization, he adds.

Next steps for the team include fabricating usable carbon microstructures, as well as charring the resins at temperatures above the 800 degrees Celsius tested in this study. Moving to higher temperatures may destroy the microstructures, Maruo says, but there is a chance they will turn the surfaces into graphite, a higher-quality conductor than the carbonized surfaces they have created so far.